Frog sciatic nerves have been exposed to both continuous wave (CW) and pulse microwave radiation. Fatigue or loss of vitality (the ability of the nerve to continue firing under rapid stimulation) have been investigated using microwaves of 2.45 GHz frequency. Distinct changes in the vitality and refractoriness of the exposed nerves were seen in comparison to control nerves for specific absorption rates (SAR's) of 10 mW/g and above. No differences in rundown time were observed between the continuous wave and pulse wave exposure using the same average SAR. No difference in loss of vitality was observed for synchronous microwave pulses with the peak of the compound action potential, quiescent period, or asynchronous pulses which occurred at random periods in relationship to the compound action potentials. Exposures of frog sciatic nerves to sine-wave modulated 2.45 GHz microwaves at frequencies of 16 and 32 Hz have been completed. A higher average SAR was required to produce a loss of vitality in the exposed nerve then was necessary for CW and pulse microwaves. The loss in vitality was the same for all modulation frequencies.